This invention relates to analogs of methionine.sup.5 -enkephalin and leucine.sup.5 -enkephalin wherein the L-phenylalanyl residue in position 4 has been substituted by various amino acid residues. Enkephalin, a naturally occurring pentapeptide, has been isolated and found to be a mixture of two pentapeptides which differ only in the fifth amino acid residue. Leucine.sup.5 -enkephalin is thus represented by the following structural formula ##STR1## and methionine.sup.5 -enkephalin by the following formula ##STR2## wherein the Tyr, Phe, Met and Leu residues are all of the L-stereochemical configuration.